Adding a new band to the iPhone may sound like a simple matter, but it’s a much more difficult task than it appears. In an interview with GigaOM, Barry Matsumori, chief marketing and strategy officer for adaptive antenna maker Ethertronics, explained why.

The problem of handset design

Vendors like Apple simply can’t slap a new antenna into a handset and start shipping. There’s about a $1 cost per device when adding a new band, which isn’t small in the low-margin world of handsets, Matsumori said — though it’s a cost Apple can more easily absorb than most. The real cost is engineering. Adding a new band requires a manufacturer to fundamentally redesign the device, building it around its new antenna configuration. “The big issue is the ripple effect it causes throughout the phone,” Matsumori said.

A new band requires not only a new antenna, but also new power amplifiers, filters and a new radio chipset – all of which have to be crammed into the ever-shrinking confines of the handset. The vast majority of space in today’s slim smartphones is dominated by the battery and display. All the other components are stuffed into the niches alongside the battery. But even if a handset designer can find the space for a new band’s components, he faces an even bigger problem: interference.

Any time you add a new emitting component like an antenna, it changes the radio frequency profile of the device, Matsumori said. Each new radio wave-emitting element can interfere with every other element in the device. A designer not only has to find room for the new antenna and supporting elements, but it has to place them in relation to other components so they don’t degrade the overall performance of the device. Ultimately, the more bands a vendor adds, the poorer signals each of those bands will transmit, Matsumori said.

The iPhone already has signal strength problems, as many iPhone 4 users discovered in 2010. That could be a reflection of the number of bands and technologies it already supports, or it could be a decision by Apple to sacrifice radio performance for industrial design. But the bottom line is Apple needs to think carefully if it wants to extend the iPhone to T-Mobile as any tinkering with the design will not only add cost, but potentially degrade performance for all its customers – not just T-Mobile’s. The next generation of smartphones will start using adaptive antenna technologies developed by companies like Ethertronics and SkyCross, which will help solve some of those interference problems. If Apple has the will, it can definitely find a way to technically fit T-Mobile into its schemes. But the even bigger obstacle T-Mobile faces to getting its sought-after iPhone would be a business one.

“It boils down to a simple business decision,” Matsumori said. “Will these guys give me enough volume to justify the expense? Or will [supporting the new band] be a strategic enough decision for me to buy in?”

When will it be T-Mobile’s turn?

Apple justified that engineering expense when it created a CDMA version of the iPhone 4 for Verizon Wireless. Verizon, however, is not only a much bigger operator than T-Mobile; the CDMA iPhone variant had much more exposure beyond Big Red. Sprint, other smaller U.S. operators and international carriers have since picked up the device. Subsequently, the new iPhone 4S supports both CDMA and GSM/HSPA, but in that case, Apple didn’t have to add any new bands to the device, just a new multi-mode radio chip. An iPhone to support T-Mobile’s HSPA+ network would have a much smaller reach — essentially T-Mobile and a few Canadian operators.

Wireless Intelligence projects 38 distinct LTE bands in 2015

When the iPhone 5 comes along, Apple could finally discover that the math adds up. T-Mobile’s Ray is right that the Apple’s next smartphone will almost certainly have AWS antennas, since AWS is where both Verizon and AT&T will launch a good hunk of their LTE networks. But that doesn’t mean Apple will support HSPA+ on that band. The antennas will be in place, but Apple will still have to procure radio chipsets that recognize T-Mobile’s network on AWS. Considering the huge number of bands and technologies the next iPhone will need — covering new LTE networks as well as legacy 4G, 3G and 2G technologies — there’s a chance T-Mobile’s specialized configuration still won’t make the cut.

In fact, Apple will find it very difficult to continue its strategy of building a single iPhone that works across global networks. My recent GigaOM Pro analysis (subscription required) explores how the fragmentation of LTE into dozens of bands across the world and within the U.S. will create a big dilemma for handset vendors who will be forced to pick and choose which LTE bands they can feasibly support in a single device. The biggest impact of those choices will be felt by the smaller carriers who don’t have the clout of Verizon to get devices made for their LTE networks.